1. Field of the Invention
The present invention relates to a sludge treatment process and an apparatus thereof and more particularly, to a multistage process and a package apparatus thereof for mainly concentration of suspended sludges but also for removal of dissolved, colloidal, suspended, volatile, and living contaminants from liquid.
2. Description of the Prior Art
Various types of sludge treatment processes are well known in the art. In such processes, the suspended contaminants in liquid sludge stream are commonly removed by a gravity thickening separation procedure wherein the particles in suspension have a specific gravity greater than that of the liquid in which they are suspended. Therefore, gravity thickening takes advantage of the difference in specific gravity between the solids and water. A gravity thickener normally consists of two truss-type steel scraper arms mounted on a hollow pipe shaft keyed to a motorized hoist mechanism. A truss-type bridge is fastened to the tank walls or to steel or concrete columns. The bridge spans the tank, and supports the entire mechanism. The thickener resembles a conventional circular clarifier with the exception of having a greater bottom slope. Slope enters at the middle of the thickener and the solids settle into a sludge blanket at the bottom. The concentrated sludge is very gently agitated by the moving rake which dislodges gas bubbles and prevents bridging of the sludge solids. It also keeps the sludge moving toward the center well from which it is removed. Supernatant liquor passes over an effluent weir around the circumference of the thickener. It has been shown that in the operation of gravity thickeners it is desirable to keep a sufficiently high flow of fresh liquid entering the concentrator to prevent septic conditions and resulting odors from developing.
When the specific gravity of the suspended contaminants is similar to that of the water, then a dissolved air flotation thickening procedure is more effective and is employed. Several types of prior art flotation processes have been developed for the separation of suspended particulates from a liquid sludge stream. In a conventional dissolved air flotation system, a recycled subnatant flow is pressurized from 30 to 70 pound per square inch and then saturated with air in a separate pressure tank. The pressurized effluent is then mixed with the influent sludge and subsequently released into the flotation tank. The excess dissolved air then separates from solution, which is now under atmospheric pressure, and the minute (average diameter 80 microns) rising gas bubbles attach themselves to particles which form the floating sludge blanket. The thickened blanket is skimmed off and pumped to the downstream sludge handling facilities while the subnatant is returned to the plant. Polyelectrolytes are frequently used as flotation aids, to enhance performance and create a thicker sludge blanket.
It has been a standard practice for engineers to construct separate liquid treatment units to remove dissolved, colloidal, suspended, volatile and living contaminants, and to construct separate gravity thickening units for heavy sludges such as primary sludges, and separate flotation thickening units for light weight activated sludges.
Neither such conventional gravity thickening systems nor conventional dissolved air flotation thickening systems are perfect for sludge concentration. An ideal sludge thickening system shall include both gravity thickening and flotation thickening.
Particularly, such conventional flotation thickening systems require separate pressure vessels for dissolving gas and for gas bubble generation. The capital costs and land space requirements of such conventional flotation thickening systems and conventional gravity thickening systems are both high.
Still such conventional flotation thickening systems are not flexible for operation. An ideal flotation thickening system shall be able to be operated under any operational modes, such as full flow pressurization, partial flow pressurization, or recycle flow pressurization.
Such conventional water and wastewater treatment processes and apparatuses thereof are described in the U.S. Pat. No. 3,171,804 to Rice, U.S. Pat. No. 3,307,701 to Krofta, U.S. Pat. No. 3,820,659 to Parlette, U.S. Pat. No. 4,022,696 to Krofta, U.S. Pat. No. 4,151,093 to Krofta, U.S. Pat. No. 4,157,952 to Krofta, U.S. Pat. No. 4,184,967 to Krofta, U.S. Pat. No. 4,303,517 to Love et al, U.S. Pat. No. 4,377,485 to Krofta, U.S. Pat. No. 4,626,345 to Krofta, U.S. Pat. No. 4,626,346 to Hall, U.S. Pat. No. 4,673,494 to Krofta, U.S. Pat. No. 4,673,498 to Swinney et al, U.S. Pat. No. 4,673,500 to Hoofnagle et al and L.K. Wang, Using Air Flotation and Filtration in Color and Giardia removal. U.S. Department of Commerce, National Technical Information Service, Springfield, Virginia, USA. Technical Report No. PB89-158398/AS. October 1988. L.K. Wang and W.J. Mahoney. Treatment of Storm Run-off by Oil-Water Separation, Flotation, Filtration and Adsorption, Part A: Wastewater Treatment. Proceedings of the 44th Industrial Waste Conference, P. 655-666, May 1989. L.K. Wang, M.H.S. Wang and W.J. Mahoney. Treatment of Storm Run-off by Oil-Water Separation, Flotation, Filtration and Adsorption: Part B: Waste Sludge Management. Proceedings of the 44th Industrial Waste COnference, P. 667-673, May 1989. L.K. Wang, and M.H.S. Wang, bubble Dynamics and Air dispersion Mechanisms of Air Flotation Process Systems, Part A: Material Balances. Proceedings of the 44th Industrial Waste Conference, P. 493-504, May 1989. M. Krofta and L.K. Wang, Application of Dissolved Air Flotation to the Lenox Massachusetts Water Supply: Sludge Thickening by Flotation or Lagoon. Journal of New England Water Works Association, P. 265-284, September 1985.